The purpose of this test is to evaluate the correct operation of an inductive style ABS sensor based upon output voltage and frequency in relation to the speed of the road wheel.
Connection for diagnostic work will vary dependent on application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice
PicoScope offers a range of options within the test kits.
Dependent on difficulty of access, choose from:
Testing sensors and actuators (to include relevant circuit/connectors):
Access to individual wheel-speed sensors may be difficult.
All wheel-speed sensors connect to the ABS control module, which is usually located in the engine bay.
Manufacturer data is required to identify the sensor pins at the ABS control module multi-plug.
PicoScope can be connected to gather waveforms from 2 or more wheel-speed sensors during a road test. In these conditions, the computer and screen must be out of the driver's view and/or operated by a second technician.
These known good waveforms have the following characteristics:
The oscillation frequency and amplitude are dependent on the wheel speed.
Example 1, Channel A has no gaps or atypical peaks or troughs with an approximately constant wheel rotational speed.
Example 2, Channel B waveform displays a periodic abnormality, causing an increased amplitude over a couple of cycles. This is typical of a rotating component issue (the sensor is fixed), e.g. indicating a damaged reluctor wheel.
Example 3, shows the waveforms from the additional connection of Channel C and Channel D to the remaining wheels, with both displaying good waveforms(Channel B remains faulty).
ABS and its derivatives, including stability and traction control, are safety-related systems fitted to vehicles, ATE, Bosch and Bendix being generally recognised manufacturers. All these systems have similar operational strategies relying on sensors to encode in their outputs the speeds of each individual wheel.
Inductive wheel speed sensors encode this output as an oscillatory waveform, with each cycle having a fixed relationship to the rotational position of the wheel.
In the examples above, the waveform cycles twenty-nine time per wheel revolution. Thus, the quicker the wheel rotates, the quicker the frequency the waveform cycles. Sensor amplitude is also proportional to wheel rotational speed, as the wheel speed increases, the waveform amplitude increases.
The ABS control module expects within tolerance oscillatory waveforms from all wheel-speed sensors. When the brakes are applied and a wheel locks, then the frequency and amplitude from that wheel will fall outside of other sensor signals. In the first instance, this triggers a release of hydraulic pressure to that wheel brake. The ABS control module will then begin to adjust the hydraulic pressure to the wheel, using internal electric pump and valve mechanisms, until a within tolerance waveform is observed or the brake pedal is released.
Traction control systems, check for individual wheel oscillation frequency increases, as might occur if a wheel loses grip and spins faster than the others. In this scenario, the engine power output is reduced until the frequencies from all sensors are equalised, indicating traction has been regained. Some systems momentarily apply the brake to the spinning wheel to aid traction through differential action. There will usually be a flashing warning lamp to indicate to the driver that traction control is taking place.
If the wheel speed signals continuously fall outside of normal parameters then the ABS function may be turned off. A driver warning lamp will be illuminated and as with any electrical fault on ABS, normal hydraulic braking is maintained.
Selection of component-related Diagnostic Trouble Codes (DTCs):
C0000 - Vehicle Speed Information Circuit Malfunction
C0035 - Left Front Wheel Speed Circuit Malfunction
C0036 - Left Front Wheel Speed Sensor Circuit Range/Performance (EBCM)
C0040 - Right Front Wheel Speed Circuit Malfunction
C0041 - Right Front Wheel Speed Sensor Circuit Range/Performance (EBCM)
C0045 - Left Rear Wheel Speed Circuit Malfunction
C0046 - Left Rear Wheel Speed Sensor Circuit Range/Performance (EBCM)
C0050 - Right Rear Wheel Speed Circuit Malfunction
C0051 - Right Rear Wheel Speed Sensor Circuit Range/Performance (EBCM)
C0221 - Right Front Wheel Speed Sensor Circuit Open
C0222 - Right Front Wheel Speed Signal Missing
C0223 - Right Front Wheel Speed Signal Erratic
C0225 - Left Front Wheel Speed Sensor Circuit Open
C0226 - Left Front Wheel Speed Signal Missing
C0227 - Left Front Wheel Speed Signal Erratic
C0229 - Drop Out of Front Wheel Speed Signals
C0235 - Rear Wheel Speed Signal Circuit Open
C0236 - Rear Wheel Speed Signal Circuit Missing
C0237 - Rear Wheel Speed Signal Erratic
C0238 - Wheel Speed Mismatch
C0245 - Wheel Speed Sensor Frequency Error
C0300 - Rear Speed Sensor Malfunction
C0305 - Front Speed Sensor Malfunction
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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